Fuel properties impact the performance and emissions behavior of diesel engines through their influence on the physical process associated with jet penetration, entrainment and fuel-air mixing, as well as by changes to the combustion chemistry associated with fuel chemistry, aromatics, molecular weight and additive concentrations. Continued focus on ultra-low nitrogen oxides (NOx) engine-out targets and variation of the available fuel on the world-wide market drives the need for a deeper understanding of the changes to the engine behavior caused by fuel property fluctuations.
Homogeneous charge compression ignition (HCCI) and related advanced combustion control strategies, such as partially pre-mixed charge compression ignition (PCCI), can be used to affect low temperature combustion (LTC) in compression ignition engines. These LTC modes of engine operation allow the possibility of low fuel consumption and low exhaust emissions; particularly NOx and smoke or particulate matter. The LTC mode is characterized by at least a fraction of the air-fuel mixture being a homogeneous vapor and initial heat release taking place at relatively low temperature. Operation in LTC mode has local equivalence ratios of about 0.2 to 2.0 and local temperatures of 1500 to 2100 Kelvin.
Several studies to assess the effect of diesel fuel property changes on engine-out emissions have been reported. Many of these suggest conflicting results on the directional influences of critical fuel properties on engine behavior, some of which is explained by the significant differences in NOx levels and engine operating conditions under which the data was gathered. Through experiments on a HCCI engine, Bunting, B. G., Crawford, R.; Wolf, L.; and Xu, Y. “The Relationships of Diesel Fuel Properties, Chemistry, and HCCI Engine Performance as Determined by Principal Component Analysis,” SAE Paper No. 2007-01-4059, Powertrain and Fluid Systems Conference and Exhibition, Chicago, Ill., October 2007, found that indicated fuel consumption is controlled by the fuel energy content whereas ignition characteristics are influenced by cetane number and that fuel and engine characteristics must be matched to achieve optimum performance. The problem is complicated by the typically high degree of confounding between fuel properties, which make it difficult to isolate individual effects. For example, Rosenthal, M. L. Bendinsky, T., “The Effects of Fuel Properties and Chemistry on the Emissions and Heat Release of Low-Emission Heavy Duty Diesel Engines,” SAE Paper No. 932800, Fuels and Lubricants Meeting and Exposition, Philadelphia Pa., October 1993, concluded that aromatic content is the primary fuel parameter driving NO and particulate matter (i.e., smoke) emissions. However, later, Ullman, T. L., Spreen, K. B., Mason, R. L., “Effects of Cetane Number of Emissions From A Prototype 1998 Heavy-Duty Diesel Engine,” SAE Paper No. 950251, February 1995, reported that increasing cetane decreased all regulated emissions on a heavy-duty engine.
As illustrated by the foregoing studies, there are inherent challenges in trying to reduce NOx emissions at minimum fuel consumption by attempting to characterize fuel effects on engine behavior and the extent to which operating conditions and the combustion system may influence the relative trends. Quantifying the relative significance of fuel properties such as cetane number, distillation curves, aromatic concentration, besides others, on a wide range of diesel engines is rather difficult. While past efforts have been instrumental in providing reasonable insights into fuel effects on generally high NOx engines and HCCI systems, the available literature on advanced ultra-low NO combustion systems which do not employ HCCI combustion technology, and using ultra-low sulfur diesel (ULSD) (less than 15 ppm sulphur content) fuel, is rather limited.
Therefore, it is highly desirable to develop an effective method for reducing NOx emissions from diesel engines operating in a low temperature combustion mode at minimum fuel consumption. It would also be desirable to provide a diesel fuel composition which effectively reduces NOx emissions from diesel engines operating in a low temperature combustion mode at minimum fuel consumption.